1. Field of the Invention
The present invention relates to an apparatus for controlling power generation for a vehicle mounted on an automobile, truck, and the like.
2. Description of the Related Art
A power generation controller for a vehicle detects the voltage at a control terminal (i.e., an output terminal or the positive terminal of a battery) and compares the detected voltage with a reference value. When the voltage at the control terminal is higher than the reference value, the power generation controller turns OFF a transistor that controls the ON/OFF state of the field current. On the other hand, when the voltage is lower than the reference value, the power generation controller turns the transistor ON. As a result, the power generation controller controls the duty cycle of the field winding and controls the voltage at the control terminal to maintain the voltage at a constant value.
When an electrical load is electrically connected, electric current of an amount corresponding to the connected electrical load is drawn from the battery. The amount by which the voltage drops as a result of the electric current being drawn is mitigated such that the voltage at the control terminal is maintained by increasing the duty cycle of the field winding and increasing the amount of generated power. The control is performed instantly. Therefore, the voltage drop at the control terminal is minimal, and the voltage is maintained at an almost constant value. However, at this time, the power generation torque (driving torque) increases because the field current of the power generator suddenly increases. As a result, engine rotation frequency decreases. Particularly in an idling rotation range, the engine may stall as a result of the decrease in the engine rotation frequency. To prevent the engine from stalling, a following method is known as a conventional technology, as disclosed in Japanese Patent Laid-open Publication No. 05-300669. In this method, the rate at which the duty cycle of the field winding increases is restricted when the field current increases, thereby suppressing a sudden increase in the power generation torque of the power generator. This method suppresses power generation by the power generator. Therefore, torque variations can be suppressed. However, while this function is performed, the amount of generated power becomes insufficient and output voltage from the power generator drops.
When the rotation frequency decreases while the output current from the power generator is constant, the field current is increased to maintain output current. Whether the rate at which the field current increases is suppressed when the rotation frequency decreases, as shown in Japanese Patent Laid-open Publication No. 05-300669, is decided by whether the amount of increasing the field current to be required is greater than a limit value of the rate at which the field current increases. In other words, when the degree of decrease in the rotation frequency is large, the amount of increase in the field current required to offset the decrease in the rotation frequency becomes large. Under these circumstances, it is likely that the increase in field current will exceed the threshold. It is therefore likely that suppression of the rate of increase will be triggered.